Internal combustion engines may use a variable camshaft timing (VCT) system to improve fuel economy and emission performance of a vehicle. The VCT system may be coupled to the intake and/or exhaust valve for advancing or retarding valve lift events. As an example, in an oil pressure actuated device, the VCT system may include an oil control valve (OCV) for adjusting an angular position (or camshaft position) of a camshaft phaser relative to the camshaft. The OCV may be actuated by an actuator controlled with a camshaft duty cycle based on a desired camshaft timing. The camshaft duty cycle needs to be closely controlled to meet the desired camshaft timing.
Other attempts to control the camshaft timing include adjusting a control signal to the VCT system based on feedback of a camshaft position. One example approach is shown by Simpson et al. in U.S. Pat. No. 6,571,757. Therein, a VCT phaser is activated by a spool valve. The spool valve position is controlled based on a feedback from a VCT phase measurement via a sensor.
However, the inventors herein have recognized potential issues with such systems. As one example, under certain conditions, the spool valve position may not be effectively controlled based on the feedback of the VCT phase measurement due to a low sampling rate of the VCT phase. The VCT phase, or the camshaft position, may be sampled when a camshaft trigger wheel edge on the camshaft phaser passes a camshaft position sensor. As the camshaft phaser, together with the camshaft trigger wheel edge, rotating with the camshaft relative to the non-rotating camshaft position sensor, the camshaft position is sampled discretely. The sampling period of the camshaft position is determined by both the engine speed and the number of camshaft trigger wheel edges on the camshaft phaser. For example, in a typical four-stroke engine VCT system, the sampling period T2 of the camshaft position may be expressed as:
            T      ⁢                          ⁢      2        =                  60        ×        2                              ω          crank                ×                  N          edges                      ,wherein ωcrank denotes engine speed in RPM, and Nedges denotes the number of camshaft trigger wheel edges. During low engine speed or when the rate of engine speed change is high, the camshaft position sampling period may be too long to effectively control the camshaft timing to meet the dynamic changes in the engine operating condition.
In one example, the issues described above may be addressed by a method comprising adjusting a camshaft phaser with a camshaft duty cycle determined based on a sampled camshaft position; and adjusting the camshaft phaser with an estimated camshaft position determined based on the camshaft duty cycle between sampling the camshaft position. In this way, the VCT system may be controlled with a sufficiently high frequency camshaft duty cycle signal at a greater range of engine operating conditions.
As one example, the camshaft timing may be adjusted by actuating the oil control valve of the VCT system with a camshaft duty cycle signal. If the engine speed is higher than a threshold, the camshaft duty cycle may be adjusted based on feedback of the sampled camshaft position and independent of an estimated camshaft position. If the engine speed is lower than the threshold, the camshaft duty cycle signal may be adjusted based on the sampled camshaft position and the estimated camshaft position, with the estimated camshaft position intermediate consecutive sampled camshaft positions. The estimated camshaft position may be calculated based on the most recent camshaft duty cycle signal via a model of the VCT system. The estimated camshaft position may predict the camshaft position between the actual camshaft position sampling instants. As such, the response time of the VCT control may be reduced, and system performance during transient operating conditions may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.